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414                         METALLURGY OF IRON AND STEEL.
Another function which may play a part in the operation is the increase in capacity to dissolve or occlude gases, and as far as the value of the casting is concerned this will be equivalent to destroying them. It is not known how far this determines the situation, but it is evident that it has no connection with the power to unite with oxygen. It was once thought that aluminum increased the fluidity of steel by lowering the point of fusion, but experiments with a Le Chatelier pyrometer* gave the same melting point of 1475° C. for ordinary soft steel as for an alloy with five per cent, of aluminum. The tendency of both aluminum and silicon is to make the steel sluggish; such metal will run through small passages without chilling better than ordinary steel, as the latter foams and froths when in contact with cold surfaces, and the flow is thereby impeded and sufficient surface exposed to chill the advance guard of the stream.
The percentage of manganese should not exceed .70 in soft castings nor .80 in harder steels, since more than this may render the metal liable to crack under shock. Silicon can be present up to .10 per cent, in the mild steels and .35 per cent, in the hard without any diminution in toughness. Aluminum is seldom present except in traces, and should not be over .20 per cent., for it decreases the ductility. The carbon must vary according to the desired tensile strength and the use to which the casting is to be put; when over .70 per cent, the steel becomes so hard that machining is slow, and there is danger of lines of weakness from shrinkage in complicated shapes.
SEC. XXf.—Physical tests on soft steel castings.—Since the failure of cast-work is almost always due to sudden strain, it is the safer plan to have the metal for common purposes between .30 and .50 per cent, in carbon, but when great toughness is required it should not be over .15 per cent. This latter specification also presupposes a low content of manganese, silicon, and, above all, of phosphorus; with this composition the casting displays all the characteristics usually associated with the toughest of rolled shapes. A test on an unannealed gear-wheel of such metal, manufactured by The Pennsylvania Steel Co., was made by cutting the rim between the spokes and then bending one arm to a right angle, twisting another through more than 180° without sign of fracture, while a
* See article on Pyrometric Data, by H. M. Howe, Engineering and Mining Journal, October 11.1890, p. 426.